Substrate and production method therefor

ABSTRACT

The object of the present invention is to provide a sintered aluminum nitride substrate which has a via hole and an internal electrically conductive layer, having high thermal conductivity and high adhesion strength between the sintered aluminum nitride substrate and the internal electrically conductive layer or the via hole and having other excellent properties. The substrate according to the invention comprising an internal electrically conductive layer, at least one electrically conductive via hole formed between the internal electrically conductive layer and at least one surface of the substrate, wherein the thermal conductivity of the aluminum nitride sintering product at 25° C. is 190 W/mK or more, and the adhesion strength between the aluminum nitride sintering product and the internal electrically conductive layer is 5.0 kg/mm 2  or more.

TECHNICAL FIELD

[0001] The present invention relates to a substrate having inside anelectrically conductive layer (internal electrically conductive layer),having an electrically conductive via hole formed between the internalelectrically conductive layer and the surface of the substrate, which isformed from an aluminum nitride sintering product, and relates to aprocess for producing the substrate. More particularly, the inventionrelates to the above-mentioned substrate which has high thermalconductivity, good adhesion properties between the aluminum nitridesintering product and the internal electrically conductive layer or theelectrically conductive via hole and high denseness of the internalelectrically conductive layer and the electrically conductive via holeand hardly suffers warpage, and relates to a process for producing thesubstrate.

BACKGROUND ART

[0002] Since aluminum nitride sintering products have excellentproperties such as high thermal conductivity, good electrical insulationproperties and a coefficient of thermal expansion almost equal to thatof Silicon (Si) for forming integrated circuits, they are used assubstrates (sometimes referred to as “aluminum nitride substrates”hereinafter) on which semiconductor circuit parts are mounted.Particularly, aluminum nitride substrates having a so-calledelectrically conductive via hole (sometimes referred to as a “via hole”simply hereinafter) that is a through hole filled with an electricallyconductive material enable electrical connection between externalcircuits of a semiconductor through the via hole, and hence they areextremely useful.

[0003] In recent years, miniaturization of semiconductor circuitmanufactured articles and improvement of performance thereof have beenpromoted, and with such promotion, the number of the via holes formed inthe aluminum nitride substrates has been increased and the arrangementof the via holes has been complicated. To meet such market requirements,there has been utilized such an aluminum nitride substrate (1) as shownin the sectional view of FIG. 1, which has inside an internalelectrically conductive layer (2) and plural electrically conductive viaholes (3) which are electrically connected to one another through theinternal electrically conductive layer. The aluminum nitride substratehaving an electrically conductive layer inside can be produced bylaminating plural aluminum nitride molded products having via holesthrough electrically conductive paste layers and then dewaxing andsintering the resulting aluminum nitride molded product laminate.

[0004] The aluminum nitride sintering product produced by the aboveprocess, however, has a thermal conductivity of at most about 170 W/mKat 25° C. because of restriction imposed by that sintering of theelectrically conductive layer and sintering of the substrate aresimultaneously carried out.

[0005] On the other hand, as one method to calcine aluminum nitride(simple substrate) having no via hole and no conductive layer, atwo-step firing method wherein the sintering temperature is changed inthe specific range is carried out (Japanese Patent Laid-Open PublicationNo. 105525/1993). In this method, a high thermal-conductive aluminumnitride sintering product having a thermal conductivity of about 200W/mK at 25° C. can be obtained.

[0006] When the sintered aluminum nitride substrate having the via holeand the internal electrically conductive layer is prepared according tothe above-mentioned two-step firing method, however, it was difficult toobtain sufficiently high adhesion strength between the aluminum nitridesintering product and the internal electrically conductive layer.Further, there was a problem that the electrically conductive layer didnot have satisfactory denseness and as a result cracks occurred insidethe aluminum nitride substrate or the value of resistance of the viahole was increased. Moreover, there was a problem of large warpage ofthe substrate.

[0007] Accordingly, it has been desired to develop an aluminum nitridesubstrate with a via hole and an internal electrically conductive layer,which has a high thermal conductivity of the aluminum nitride sinteringproduct, high adhesion strength of the internal electrically conductivelayer or the via hole to the aluminum nitride sintering product andother excellent properties.

DISCLOSURE OF THE INVENTION

[0008] The present inventor has earnestly studied to solve theabove-mentioned problems. As a result, it has been found that thedefects of the substrate are caused by the action of carbon afterdewaxing, and it has been further found the followings: cracks occurringinside the aluminum nitride substrate and increase of warpage of thesubstrate can be inhibited by controlling the carbon residue of thedewaxed aluminum nitride molded product laminate to the specific range;the adhesion strength between the aluminum nitride sintering product andthe internal electrically conductive layer or the via hole can besufficiently enhanced and stabled by controlling the carbon residue ofthe dewaxed laminate, the composition of the electrically conductivepaste used and the temperature range of the two-step firing method tothe specific ranges; and the thermal conductivity of the aluminumnitride sintering product can also be sufficiently enhanced by theseoperations. Based on the finding, the present invention has beenaccomplished.

[0009] The substrate according to the invention is a substrate having aninternal electrically conductive layer, at least one electricallyconductive via hole formed between the internal electrically conductivelayer and at least one surface of the substrate, which is formed from analuminum nitride sintering product, wherein:

[0010] the thermal conductivity of the aluminum nitride sinteringproduct at 25° C. is 190 W/mK or more, and the adhesion strength betweenthe aluminum nitride sintering product and the internal electricallyconductive layer is 5.0 kg/mm² or more.

[0011] In the substrate of the invention, the adhesion strength betweenthe aluminum nitride sintering product and the electrically conductivevia hole is preferably 5.0 kg/mm² or more.

[0012] In the substrate of the invention, it is preferable that theelectrically conductive via hole comprises a sintering product of anelectrically conductive paste having a refractory metal concentration of85 to 95% by weight and the internal electrically conductive layercomprises a sintering product of an electrically conductive paste havinga refractory metal concentration of 65 to 83% by weight.

[0013] A metallized substrate according to the invention has anelectrically conductive pattern formed on at least one surface of bothsurfaces of the above-mentioned substrate, wherein at least a part ofthe electrically conductive pattern is electrically connected to theelectrically conductive via hole.

[0014] The process for producing a substrate according to the inventioncomprises:

[0015] forming at least one via hole-forming through hole in a firstaluminum nitride molded product comprising an aluminum nitride powder, asintering aid and an organic binder,

[0016] filling the through hole with an electrically conductive paste(A) comprising 100 parts by weight of a refractory metal powder and 2 to10 parts by weight of an aluminum nitride powder,

[0017] coating the surface of the first aluminum nitride molded productwith an electrically conductive paste (B) comprising 100 parts by weightof a refractory metal powder and 2 to 20 parts by weight of an aluminumnitride powder to form an electrically conductive paste layer,

[0018] laminating a second aluminum nitride molded product comprising analuminum nitride powder, a sintering aid and an organic binder on thefirst aluminum nitride molded product through the layer of theelectrically conductive paste (B), and

[0019] dewaxing the resulting aluminum nitride molded product laminateso that the carbon residue becomes 800 to 3000 ppm, then sintering thelaminate at a temperature of 1200 to 1700° C. and further sintering thelaminate at a temperature of 1800 to 1950° C.

[0020] In the above process, it is preferable that the concentration ofthe refractory metal in the electrically conductive paste (A) with whichthe via hole-forming through hole of the first aluminum nitride moldedproduct is to be filled is in the range of 85 to 95% by weight and theconcentration of the refractory metal in the electrically conductivepaste (B) with which the surface of the first aluminum nitride moldedproduct is to be coated is in the range of 65 to 83% by weight.

[0021] It is particularly preferable that the viscosity of theelectrically conductive paste (A) with which the via hole-formingthrough hole of the first aluminum nitride molded product is to befilled is in the range of 100 to 30000 poise at 25° C./5 rpm and theviscosity of the electrically conductive paste (B) with which thesurface of the first aluminum nitride molded product is to be coated isin the range of 800 to 1200 poise at 25° C./5 rpm.

[0022] According to the process of the invention, the aforesaidsubstrate of the invention can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a schematic sectional view of a substrate, in a typicalembodiment, according to the present invention.

[0024] FIGS. 2 to 4 are schematic sectional views of substrates, inother embodiments, according to the present invention.

[0025]FIG. 5 is a schematic perspective view of a substrate, in anotherembodiment, according to the present invention.

[0026]1: substrate

[0027]2: internal electrically conductive layer

[0028]3: electrically conductive via hole

BEST MODE FOR CARRYING OUT THE INVENTION

[0029] The substrate of the invention comprising an aluminum nitridesintering product has an electrically conductive layer formed inside.Although the thickness of the internal electrically conductive layer isnot specifically restricted, it is usually in the range of 5 to 50 μm.The material to constitute the internal electrically conductive layer isnot specifically restricted provided that it is a refractory metal. Thematerial is usually a refractory metal such as tungsten or molybdenumand is preferably one containing aluminum nitride in an amount of 2 to20 parts by weight based on 100 parts by weight of the refractory metal.

[0030] The internal electrically conductive layer is usually formed as aflat film inside the substrate. Although this layer is generally formedin parallel to both surfaces of the substrate, it may be provided as alayer inclined to the substrate surface to a certain extent whennecessary.

[0031] The internal electrically conductive layer does not need to beprovided all over the horizontal section of the aluminum nitridesubstrate, and it may be provided partially. It is particularlypreferable to form the internal electrically conductive layer as adesired circuit pattern according to the positions and the number of theelectrically conductive via holes to be formed. The proportion of theinternal electrically conductive layer to the horizontal section of thealuminum nitride substrate is desired to be in the range of usually 30to 100%.

[0032] In the present invention, two or more internal electricallyconductive layers may be provided at prescribed intervals inside thealuminum nitride sintering product. In this case, at least one via holeis generally formed between the internal electrically conductive layersto electrically connect those layers to each other.

[0033] In the aluminum nitride substrate of the invention, at least onevia hole to link (electrically connect) the internal electricallyconductive layer to at least one surface of the substrate is formed. Thevia hole may be formed between the internal electrically conductivelayer and one surface of the substrate or may be formed with penetratingthe upper and the lower surfaces of the substrate through the internalelectrically conductive layer.

[0034] When plural via holes are formed in the substrate of theinvention, via holes corresponding to the pattern of the internalelectrically conductive layer are electrically connected to one anotherthrough the internal electrically conductive layer. Although the numberof via holes is not specifically restricted, the proportion of the totalvolume of all the via holes to the volume of the whole aluminum nitridesintering product containing the via holes is preferably in the range of0.1 to 20%.

[0035] Although the size of the via hole is not specifically restricted,the diameter is preferably in the range of 0.03 to 0.50 mm, and theratio of the length to the diameter (length/diameter) is preferably notmore than 40.

[0036] The electrically conductive material filled in the via hole isnot specifically restricted provided that it is a refractory metal.Usually, a high-melting point such as tungsten or molybdenum isemployed. The electrically conductive material is preferably onecontaining aluminum nitride in an amount of 2 to 10 parts by weightbased on 100 parts by weight of the refractory metal.

[0037] Typical embodiments of the substrate of the invention having aninternal electrically conductive layer and a via hole formed thereinare, for example, those shown in the sectional views of FIGS. 1 to 4.

[0038] The aluminum nitride substrate of the invention has strikingcharacteristics that the thermal conductivity of the aluminum nitridesintering product is not less than 190 W/mK and the adhesion strengthbetween the aluminum nitride sintering product and the internalelectrically conductive layer is 5.0 kg/mm² or more.

[0039] Since the aluminum nitride substrate of the invention is of aso-called composite system containing a via hole and an internalelectrically conductive layer, it is difficult to accurately evaluatethe thermal conductivity of the substrate itself in many cases. In thepresent invention, therefore, a thermal conductivity of an aluminumnitride substrate of the same thickness, which is made of the samematerial by the same batchwize dewaxing and sintering but has no viahole and no internal electrically conductive layer, is employed as thethermal conductivity of the aluminum nitride sintering product of theinvention. If the thermal conductivity of the aluminum nitride sinteringproduct portion can be directly measured using a residual portionobtained by removing the internal electrically conductive layer and thevia hole from the aluminum nitride sintering product by grinding or thelike, it is a matter of course to take the measured value as the thermalconductivity. In the present invention, the thermal conductivity is avalue measured at 25° C.

[0040] In the present invention, the adhesion strength between thealuminum nitride sintering product and the internal electricallyconductive layer is measured in the following manner. The surface of thesubstrate is ground until the internal electrically conductive layer isexposed outside, and on the thus exposed internal electricallyconductive layer, a thin film of Ti/Pt/Au is formed. Then, the substrateis cut to give a chip of about 5 mm×5 mm. Onto the thin film of thechip, a pin of 0.5 mmØ with a flat tip is soldered perpendicularly.Then, the pin is pulled in the perpendicular direction to measure abreaking strength. In this measurement, whether the via hole underliesthe internal electrically conductive layer onto which the pin is to besoldered have little influence on the value of the adhesion strengthirrespective of the proportion of the underlying via hole to theinternal electrically conductive layer.

[0041] The above-mentioned measurement is carried out by appropriatelyselecting the position of the pin at which the internal electricallyconductive layer is present all over the lower surface of the pin to besoldered. If the internal electrically conductive layer is formed bysuch a fine circuit pattern that the measurement by the selection of theabove position is difficult, conversion based on the area of theinternal electrically conductive layer actually evaluated has to bemade.

[0042] It has been heretofore difficult to satisfy both of increase ofthermal conductivity of the aluminum nitride sintering product andincrease of adhesion strength between the aluminum nitride sinteringproduct and the internal electrically conductive layer. In the substrateof the invention, however, the thermal conductivity of the aluminumnitride sintering product is 190 W/mK or more and the adhesion strengthbetween the aluminum nitride sintering product and the internalelectrically conductive layer is 5.0 kg/mm² or more, so that the valuesof those properties are both high. Moreover, by selecting the productionconditions from more preferred ranges, a substrate wherein the thermalconductivity of the aluminum nitride sintering product is 200 W/mK ormore and the adhesion strength is 7.0 kg/mm² or more, particularly 10.0kg/mm² or more, can be obtained.

[0043] In the substrate according to the invention, the adhesionstrength between the aluminum nitride sintering product and theelectrically conductive via hole is preferably 5.0 kg/mm² or more, morepreferably 7.0 kg/mm² or more, particularly preferably 10.0 kg/mm² ormore.

[0044] The adhesion strength between the aluminum nitride sinteringproduct and the electrically conductive via hole stands for a breakingstrength measured by cutting the substrate across the center of thevia-hole, polishing the cut surface like a mirror surface, forming athin Ti/Pt/Au film on the cut surface, soldering a pin of a diameter of0.5 mm having a flat end in a manner to come in contact with the surfaceof the via-hole perpendicularly thereto, and pulling the pin from adirection perpendicular thereto.

[0045] Next, the process for producing an aluminum nitride substrateaccording to the invention is described. The aluminum nitride substrateof the above structure may be a substrate produced by any process, butit is preferably obtained by a process comprising:

[0046] forming at least one via hole-forming through hole in a firstaluminum nitride molded product comprising an aluminum nitride powder, asintering aid and an organic binder,

[0047] filling the through hole with an electrically conductive paste(A) comprising 100 parts by weight of a refractory metal powder and 2 to10 parts by weight of an aluminum nitride powder,

[0048] coating the surface of the first aluminum nitride molded productwith an electrically conductive paste (B) comprising 100 parts by weightof a refractory metal powder and 2 to 20 parts by weight of an aluminumnitride powder to form an electrically conductive paste layer,

[0049] laminating a second aluminum nitride molded product comprising analuminum nitride powder, a sintering aid and an organic binder on thefirst aluminum nitride molded product through the layer of theelectrically conductive paste (B), and

[0050] dewaxing the resulting aluminum nitride molded product laminateso that the carbon residue becomes 800 to 3000 ppm, then sintering thelaminate at a temperature of 1200 to 1700° C. and further sintering thelaminate at a temperature of 1800 to 1950° C.

[0051] There is no specific limitation on the aluminum nitride powder toconstitute the first and the second aluminum nitride molded products,and any of commonly known ones is employable. In particular, a powderhaving an average particle diameter, as measured by a sedimentationmethod, of not more than 5 μm is preferably employed, and a powderhaving an average particle diameter of not more than 3 μm is morepreferably employed, and a powder having an average particle diameter of0.5 to 2 μm is most preferably employed. Further, an aluminum nitridepowder having an average particle diameter D1, as calculated using thespecific surface area, and an average particle diameter D2, as measuredby a sedimentation method, which satisfy the following formulas ispreferably employed, because such an aluminum nitride powder can reducea linear shrinkage ratio in the firing process and thereby improvedimensional stability of the sintering product or bring the linearshrinkage ratio close to that of the internal electrically conductivelayer, whereby the adhesion strength between the aluminum nitridesintering product and the via hole or the internal conductiveelectrically conductive layer can be further enhanced.

0.2 μm≦D1≦1.5 μm

D2/D1≦2.60

[0052] Also preferable is an aluminum nitride powder which has an oxygencontent of not more than 3.0% by weight and a cation impurity content ofnot more than 0.5% by weight with the proviso that the composition ofthe aluminum nitride is AlN, and particularly preferable is an aluminumnitride powder having an oxygen content of 0.4 to 1.0% by weight, acation impurity content of not more than 0.2% by weight and a totalamount of Fe, Ca, Si and C among the cation impurities being not morethan 0.17% by weight. When such an aluminum nitride powder is used, thethermal conductivity of the resulting aluminum nitride sintering productis greatly improved, so that such an aluminum nitride powder ispreferably used.

[0053] As the sintering aid added to the aluminum nitride molded productin the present invention, commonly known sintering aid can be usedwithout any restriction. Specifically, an alkaline earth metal compound,e.g., an oxide such as calcium oxide, or a compound comprising yttriumor a lanthanide element, e.g., an oxide such as yttrium oxide, ispreferably used.

[0054] Likewise, as the organic binder used for the aluminum nitridemolded product in the present invention, commonly known organic binderscan be used without any restriction. Examples of the organic bindersinclude acrylic resins, such as polyacrylic esters and polymethacrylicesters; cellulose resins, such as methyl cellulose, hydroxymethylcellulose, nitrocellulose and cellulose acetate butyrate; vinylgroup-containing resins, such as polyvinyl butyral, polyvinyl alcoholand polyvinyl chloride; hydrocarbon resins, such as polyolefins; andoxygen-containing resins, such as polyethylene oxide. These resins areused singly or in combination of two or more kinds. Of these, theacrylic resins are preferably used because they have good dewaxabilityand can reduce resistance of a via hole. As other components such as asolvent, a dispersant and a plasticizer, those commonly known canlikewise be used without any restriction.

[0055] As for the proportions of the components for constituting thefirst and the second aluminum nitride molded products, commonly knownproportions can be adopted without any restriction in the presentinvention. For example, the sintering aid in an amount of 0.01 to 10parts by weight and the organic binder in an amount of 0.1 to 30 partsby weight are preferably used based on 100 parts by weigh of aluminumnitride. Particularly, the sintering aid in an amount of 2 to 7 parts byweight is preferably adopted since such amount is advantageous in theenhancement of the thermal conductivity. Likewise, there is no specificlimitation on the process for producing an aluminum nitride moldedproduct using these components, and in general, the molded product isformed as a green sheet by a doctor blade method. The green sheet may beused singly, or plural green sheets may be laminated together to give aunited one having a desired thickness, prior to use.

[0056] In the present invention, each of the electrically conductivepastes used for forming the via hole and the internal electricallyconductive layer contains the aforesaid type of the refractory metalpowder. As the refractory metal powder for the electrically conductivepaste (A) for forming the via hole, a powder having an average particlediameter, as measured by the Fischer's method, of 1 to 2.5 μm ispreferably used, and a powder having an average particle diameter of 1.6to 2.0 μm is most preferably used, because occurrence of cracks insidethe via hole can be effectively inhibited. As the refractory metalpowder for the electrically conductive paste (B) for forming theinternal electrically conductive layer, a powder having an averageparticle diameter, as measured by the Fischer's method, of 0.8 to 5.0 μmis preferably used, and a powder having an average particle diameter of1.0 to 3.0 μm is most preferably used, because a dense internalelectrically conductive layer is formed and occurrence of warpage of thealuminum nitride substrate can be effectively reduced.

[0057] As the aluminum nitride powder used for the both electricallyconductive pastes, an aluminum nitride powder commonly known can be usedwithout any restriction. Particularly, the aluminum nitride powder ofthe aforesaid properties that is preferably used for the aluminumnitride molded product is preferably used, because not only it has goodsintering properties with the refractory metal powder and therebyexhibits an effect to enhance adhesion properties of the internalelectrically conductive layer but also it reduces a difference in theshrinkage ratio between the aluminum nitride portion and the internalelectrically conductive layer portion to improve dimensional stabilityof the sintering product.

[0058] In the present invention, the electrically conductive paste (A)for forming the electrically conductive via hole has a compositioncontaining 2 to 10 parts by weight of the aluminum nitride powder basedon 100 parts by weight of the refractory metal powder. If the amount ofthe aluminum nitride powder is less than 2 parts by weight in theelectrically conductive paste, the adhesion strength between the viahole and the aluminum nitride sintering product becomes low or thedifference in the shrinkage ratio between the via hole portion and thealuminum nitride sintering product portion is increased to bring about agap in the junction interface.

[0059] If the amount of the aluminum nitride powder is more than 10parts by weight, the viscosity of the electrically conductive pastebecomes high to deteriorate filling properties of the paste, and as aresult, adhesion strength between the via hole and the aluminum nitridesintering product is lowered by the voids produced, or discoloration dueto aluminum nitride is liable to take place on the surface of the viahole to increase the value of resistance. When the amount of thealuminum nitride powder is in the range of 3 to 7 parts by weight, thedifference in the firing shrinkage ratio between the via hole and theceramic is extremely small, so that the stress generated around the viahole is small and the electrical resistance of the via hole can bedecreased. Hence, such amounts are preferable.

[0060] On the other hand, the electrically conductive paste (B) forforming the internal electrically conductive layer has a compositioncontaining 2 to 20 parts by weight of the aluminum nitride powder basedon 100 parts by weight of the refractory metal powder. If the amount ofthe aluminum nitride powder is less than 2 parts by weight in theelectrically conductive paste, the adhesion strength between theinternal electrically conductive layer and the aluminum nitridesintering product is lowered, or the difference in the shrinkage ratiobetween the aluminum nitride sintering product portion and the internalelectrically conductive layer portion is increased to bring aboutseparation on the junction interface. If the amount of the aluminumnitride powder is more than 20 parts by weight, the viscosity of theelectrically conductive paste becomes high to deteriorate printability.On this account, printing non-uniformity or blur takes place to loweradhesion strength between the internal electrically conductive layer andthe aluminum nitride sintering product and to increase electricalresistance of the internal electrically conductive layer itself.

[0061] If the amount of the aluminum nitride powder is more than 20parts by weight, further, storage stability of the electricallyconductive paste itself is decreased. Therefore, if the electricallyconductive paste is used after a passage of several days from thepreparation, the viscosity of the electrically conductive paste becomeshigh to deteriorate printability, and as a result, adhesion strengthbetween the internal electrically conductive layer and the aluminumnitride sintering product is lowered by the printing non-uniformity orblur produced. In the electrically conductive paste for forming theinternal electrically conductive layer, the amount of the aluminumnitride powder is in the range of preferably 2 to 18 parts by weight,more preferably 11 to 18 parts by weight, from the viewpoint of storagestability.

[0062] In addition to the refractory metal and the aluminum nitridepowder, to each of the electrically conductive pastes used for formingthe via hole and the internal electrically conductive layer aregenerally added an organic binder, an organic solvent, etc. to make thempasty. Examples of the organic binders include acrylic resins, such aspolyacrylic esters and polymethacrylic esters; cellulose resins, such asmethyl cellulose, ethyl cellulose, hydroxymethyl cellulose,nitrocellulose and cellulose acetate butyrate; vinyl group-containingresins, such as polyvinyl butyral, polyvinyl alcohol and polyvinylchloride; hydrocarbon resins, such as polyolefins; and polyethyleneoxide. Examples of the organic solvents include di-n-butyl phthalate,diethylene glycol mono-n-hexyl ether, 2-(2-butoxyethoxy)ethyl acetateand terpineol.

[0063] In the electrically conductive paste (A) for forming the viahole, the concentration of the refractory metal is in the range ofpreferably 85 to 95% by weight, more preferably 87 to 93% by weight,from the viewpoints of enhancement of the adhesion strength of the viahole and inhibition of occurrence of cracks inside the via hole. In theelectrically conductive paste (B) for forming the internal electricallyconductive layer, the concentration of the refractory metal is in therange of preferably 65 to 83% by weight, more preferably 72 to 83% byweight, from the viewpoint of obtaining higher adhesion strength of theresulting electrically conductive layer to the aluminum nitridesintering product.

[0064] The electrically conductive paste (A) for forming the via hole isdesirably prepared so as to have a viscosity of preferably 100 to 30000poise, more preferably 500 to 7000 poise, at 25° C./5 rpm, from theviewpoints of enhancement of adhesion strength of the via hole andinhibition of occurrence of cracks inside the via hole. The electricallyconductive paste (B) for forming the internal electrically conductivelayer is desirably prepared so as to have a viscosity of preferably 800to 1200 poise, more preferably 850 to 1000 poise, at 25° C./5 rpm, fromthe viewpoint of obtaining higher adhesion strength of the resultingelectrically conductive layer to the aluminum nitride sintering product.In the preparation of these electrically conductive pastes, othercomponents commonly known, such as a dispersant and a plasticizer, areused without any restriction.

[0065] In the present invention, a through hole formed in the firstaluminum nitride molded product is filled with the via hole-formingelectrically conductive paste (A) to obtain an aluminum nitride moldedproduct having a via hole-forming through hole filled with the paste(A). There is no specific limitation on the method to form a throughhole in the aluminum nitride molded product, and a method generallyused, such as metal mold punching method or a method of using a punchingmachine, is employed. The diameter of the through hole is preferably inthe range of 0.05 to 0.50 mm in consideration of a desired hole diameterof a via hole formed in the substrate.

[0066] For filling the through hole with the electrically conductivepaste (A), commonly known methods can be adopted without anyrestriction. Specifically, a printing method, the pressurizedpenetration method or the like is used. When the ratio of the length tothe diameter (length/diameter) of the through hole is larger than 2.5,the pressurized penetration method is preferably used since the fillingcan be made more easily.

[0067] In the present invention, the surface of the first aluminumnitride molded product having a through hole filled with theelectrically conductive paste is then coated with the electricallyconductive paste (B) for forming the internal electrically conductivelayer. For the coating, commonly known methods are adopted without anyrestriction. In general, coating by screen printing is preferably used.

[0068] Subsequently, on the first aluminum nitride molded product, asecond aluminum nitride molded product that is different from the firstaluminum nitride molded product is laminated with interposing theelectrically conductive paste layer formed as above. The second aluminumnitride molded product comprises the same aluminum nitride powder,sintering aid and organic binder as previously described. The secondaluminum nitride molded product may be one having a via hole-formingthrough hole or one having no via hole-forming through hole. The throughhole formed in the second aluminum nitride molded product may not befilled with no via hole-forming electrically conductive paste. Also onthe surface of the second aluminum nitride molded product, the samelayer of the electrically conductive paste (B) as previously describedmay be formed.

[0069] Further, three or more aluminum nitride molded products may belaminated with interposing therebetween the electrically conductivepaste layers. Furthermore, on one or both surfaces of the resultingaluminum nitride molded product laminate, the electrically conductivepaste layer may be provided as a surface layer. The structure of thealuminum nitride molded product laminate is properly designed accordingto the structure of the desired aluminum nitride substrate.

[0070] For laminating the aluminum nitride molded products, commonlyknown methods are adopted without any restriction. In general, themolded products are preferably laminated and united by a hot pressingmethod or a hot water isotropic pressure method.

[0071] The aluminum nitride molded product laminate obtained as aboveneeds to be dewaxed so that the aluminum nitride portion (aluminumnitride molded product except via hole portion and internal electricallyconductive layer portion) has a carbon residue of 800 to 3000 ppm,preferably 1200 to 2500 ppm. If the carbon residue is less than 800 ppm,the thermal conductivity of the aluminum nitride sintering productbecomes lower than 190 W/mK, and hence the object of the presentinvention cannot be attained. If the carbon residue exceeds 3000 ppm,the sintering properties of the refractory metal powder becomes bad, andtherefore uniform and sufficient adhesion strength between the aluminumnitride sintering product and the via hole or the internal electricallyconductive layer cannot be obtained. In addition, cracks take place inthe aluminum nitride portion or warpage of the aluminum nitridesubstrate becomes large, and hence the object of the present inventioncannot be attained.

[0072] In the dewaxing step, the dewaxing atmosphere is not specificallyrestricted except an oxidizing atmosphere such as the open air which isliable to oxidize the refractory metal. For example, an atmosphere of aninert gas such as nitrogen, argon or helium, an atmosphere of a reducinggas such as hydrogen, an atmosphere of a mixed gas thereof, anatmosphere of a moistened gas thereof, or vacuum is preferably employed.

[0073] The dewaxing temperature is appropriately selected and is in therange of usually 500 to 1200° C., preferably 800 to 1000° C. Althoughthe heating rate to attain this temperature is not specificallyrestricted, it is preferably not more than 10° C./min.

[0074] The dewaxing time is determined so that the carbon residue of themolded product after dewaxing will be in the range of 800 to 3000 ppm.Although the period of time for which the dewaxing temperature ismaintained varies to some extent depending upon such condition as thethickness of the molded product, density of the molded product,proportions of the via hole and the internal electrically conductivelayer, dewaxing temperature, etc., it is usually determined within therange of 1 to 600 minutes.

[0075] The aluminum nitride molded product laminate having been dewaxedas above (referred to as “dewaxed laminate” hereinafter) is thensintering in a non-oxidizing atmosphere or a dried reducing gasatmosphere. The non-oxidizing atmosphere is, for example, an atmosphereof a single gas such as nitrogen, argon or helium or a mixed gasthereof, or a vacuum (or reduced pressure) atmosphere. The driedreducing gas atmosphere is, for example, an atmosphere of hydrogen or amixture of hydrogen and an inert gas.

[0076] As for the temperature conditions in the firing, the dewaxedlaminate needs to be sintered at a temperature of 1200 to 1700° C.,preferably 1500 to 1650° C., in the first step, and then sintered at atemperature of 1800 to 1950° C., preferably 1820 to 1900° C., in thesecond step. If the firing temperature in the first step is lower than1200° C., reaction for removing oxygen in the aluminum nitride byreduction with carbon remaining in the dewaxed laminate hardly proceeds,and therefore the thermal conductivity of the aluminum nitride sinteringproduct becomes lower than 190 W/mK. Consequently, the object of thepresent invention cannot be attained. On the other hand, if the firingtemperature in the first step exceeds 1700° C., sintering of aluminumnitride proceeds before the reaction for removing oxygen from thealuminum nitride by reduction with carbon remaining in the dewaxedlaminate sufficiently proceeds, and as a result, oxygen is diffused anddissolved in the aluminum nitride to inhibit increase of thermalconductivity of the aluminum nitride sintering product. Consequently,the object of the present invention cannot be attained. When the firingtemperature in the first step is in the range of 1500 to 1650° C., thereduction reaction to remove oxygen proceeds effectively, so that thistemperature range is preferable.

[0077] If the firing temperature in the second step is lower than 1800°C., the aluminum nitride cannot be sufficiently sintered, and thereforethe thermal conductivity of the aluminum nitride sintering productbecomes lower than 190 W/mK. Consequently, the object of the presentinvention cannot be attained. If the firing temperature in the secondstep exceeds 1950° C., not only the adhesion strength between the viahole or the internal electrically conductive layer and the substrate islowered but also warpage of the aluminum nitride substrate becomeslarger than 200 μm. Consequently, the object of the present inventioncannot be attained. Although the heating rate to attain this temperatureis not specifically restricted, it is preferably in the range of 1 to40° C./min.

[0078] The period of time for which the above temperature is maintainedis not specifically restricted, but it is usually determined in therange of 30 minutes to 10 hours in the first step and in the range of 1minute to 20 hours in the second step. The firing in the first and thesecond steps may be carried out as one-time sintering without decreasingthe firing temperature, or may by carried out as two-time sintering bydecreasing the firing temperature between the first step and the secondstep. In consideration of the time and energy efficiency, however, thesintering is preferably carried out as one-time firing withoutdecreasing the firing temperature.

[0079] When the distance between the upper or the lower surface of thesubstrate and the internal electrically conductive layer in thesubstrate to be produced by the above process is prescribed, thesubstrate satisfying such requirement can be obtained by, for example,exposing the side end of the internal electrically conductive layeroutside on the side surface of the substrate, then abrading or grindingthe upper or the lower surface of the substrate with measuring thedistance from the upper or the lower surface of the substrate andstopping the abrading or grinding when the prescribed distance isreached.

[0080] If warpage of the substrate having the internal electricallyconductive layer is large when the above processing is carried out, thelocation of the internal electrically conductive layer cannot bemaintained accurately. In an extreme case, the internal electricallyconductive layer is exposed outside on the substrate surface after theprocessing. The internal electrically conductive layer exposed outsideon the substrate surface causes short circuit of the electricallyconductive pattern formed on the substrate surface, and for this reason,the warpage is preferably made as small as possible prior to the aboveprocessing.

[0081] The aluminum nitride substrate of the invention is usually usedfor preparing a metallized substrate having a structure wherein anelectrically conductive pattern is formed on at least one surface of theboth surfaces facing each other and at least a part of the electricallyconductive pattern is electrically connected to the via hole. In thiscase, the electrically conductive pattern may be formed on only onesurface of the substrate, or may be formed on both surfaces so thatthese surfaces are electrically connected to each other by means of viaholes which connect the upper and the lower surfaces of the substratethrough the internal electrically conductive layer.

[0082] The electrically conductive pattern in the invention is notspecifically restricted provided that it has electrical conductivity,for example, a metallic thin film or a thick film composed of a metalpowder and an inorganic or organic binder is usually used. Of suchfilms, the metallic thin film is most preferably used because of highelectrical conductivity. As the metal to constitute the metallic thinfilm, commonly known metals are used without any restriction.Specifically, titanium (Ti), chromium (Cr), molybdenum (Mo), tungsten(W), aluminum (Al), tantalum (Ta), tungsten-titanium (W—Ti),nickel-chromium (Ni—Cr) and tantalum nitride (Ta—N) are preferably usedbecause they have good adhesion to the aluminum nitride sinteringproduct.

[0083] The above metals may be used singly or in combination of two ormore kinds. The electrically conductive pattern may be formed as asingle layer or a laminate of two or more layers.

[0084] In case of a laminate of two or more layers, it is preferable touse the above-mentioned metals for the first layer which is brought intocontact with the aluminum nitride sintering product since they have goodadhesion strength to the aluminum nitride sintering product. Of thosemetals, Ti is more preferably used since high adhesion strength can bestably obtained. Although the thickness of the first layer used as anadhesive layer is not specifically restricted, it is in the range ofusually 0.01 to 10 μm, preferably 0.05 to 5 μm, from the viewpoint ofbalance between guarantee of the reliability of adhesion strength due toincrease of the film thickness and economical effect due to shorteningof the film-forming time and reduction of the materials accompanied bydecrease of the film thickness.

[0085] For the second layer laminated on the first layer, a commonlyknown metal can also be used. When the second layer is an outermostlayer in the electrically conductive pattern of a two-layer laminate, atleast one of platinum (Pt), nickel (Ni), palladium (Pd), copper (Cu),silver (Ag) and gold (Au) is preferably used because of good electricalconductivity. Of these metals, Pt, Pd, Ag or Au is more preferably usedbecause of good corrosion resistance.

[0086] When an electrically conductive pattern of a laminate of three ormore layers wherein a layer is further laminated on the second layer isused as described later, Pt, Ni, Pd, W, W—Ti or Mo having high abilityof diffusion inhibition is more preferably used for the second layer inorder to inhibit diffusion of elements of the first and the third layerand thereby ensure stable adhesion strength between the electricallyconductive pattern and the sintering product. Although the thickness ofthe second layer is not specifically restricted, it is in the range ofusually 0.01 to 10 μm, preferably 0.05 to 5 μm because of similarreasons for the first layer.

[0087] When the third layer is laminated on the second layer, a commonlyknown metal is employable for the third layer. For example, at least oneof Pt, Ni, Pd, Cu, Ag and Au is preferably used because of goodelectrical conductivity. Of these metals, Pt, Pd, Ag or Au is morepreferably used because they have excellent corrosion resistance.Although the thickness of the third layer is not specificallyrestricted, it is in the range of usually 0.05 to 10 μm from theviewpoints of stability of conductivity and balance between reliabilityand economical effect.

[0088] In order to facilitate soldering of semiconductor elements or thelike onto the outermost metal layer, a layer of at least one solder,such as a gold-tin (Au—Sn) solder, a lead-tin (Pb—Sn) solder, agold-silicon (Au—Si) solder or a gold-germanium (Au—Ge) solder, may belaminated or patterned. Further, a solder material diffusion inhibitionlayer may be provided between the outermost metal layer and the solderlayer. For the diffusion inhibition layer, Pt, Ni, Pd, W, W—Ti or Mo ispreferably used because of high ability of diffusion inhibition.

[0089] In order to maintain the prescribed value of electricalresistance, a resistor thin film pattern to electrically connectspecific patterns of the electrically conductive patterns to each otherwith the prescribed value of resistance may be formed between thespecific patterns of the electrically conductive patterns. The resistorthin film pattern is desired to have small change in the value ofresistance with time, and besides it is desired that the value ofresistance does not vary even if the temperature of the metallizedsubstrate is changed.

[0090] The type of a resistor thin film used for the resistor thin filmpattern is not specifically restricted, but preferably used is Ta—N,Ni—Cr of the like from the viewpoint of the stability of the value ofthe resistance. The composition of the alloy selected is preferably onewhich causes small change in the value of resistance with temperature.For example, a Ta₂N layer is preferably used as a layer of Ta—N type.Provided that the size of the pattern is the same, the value ofresistance becomes smaller with decrease of the film thickness of theresistor thin film. Therefore, the film thickness of the resistor thinfilm is properly determined according to the size of the desired patternand the value of resistance. The film thickness thereof is usually inthe range of 0.01 to 0.5 μm from the viewpoint of balance between thestability of the value of resistance and the economical effect.

[0091] In the electrically conductive pattern, an inductor element and acapacitor element may be formed. The inductor element can be formed by,for example, producing a coil inductor pattern. The capacitor elementcan be formed by, for example, laminating an insulating film such as afilm of tantalum pentoxide (Ta₂O₅) on the electrically conductivepattern and then further laminating an electrode film (upper electrodefilm) on the insulating film.

[0092] The metallized substrate is usually produced by the followingprocess. That is, a large-sized substrate is produced by the aboveprocess, then electrically conductive patterns are formed repeatedly onthe surface of the substrate by the following method, and the substrateis cut into a desired size to obtain plural metallized substrates.

[0093] The size of the substrate is preferably large since a greatnumber of metallized substrates with electrically conductive patternscan be formed at once. In general, a sintering product having a size of1 to 4 inch square is employed.

[0094] The substrate surface on which the electrically conductivepattern is to be formed is preferably subjected to grinding or polishingin order to enhance the adhesion strength between the electricallyconductive pattern and the substrate. There is no specific limitation onthe grinding and the polishing, and any technique commonly known isemployed. In general, lapping, polishing, barrel polishing, sandblasting, abrasion using a grinder, or the like is employed. The surfacecoarseness of the substrate varies depending upon the purpose, but it ispreferable to carry out the polishing so that the centerline averagecoarseness (Ra) becomes not more than 1.0 μm, more preferably not morethan 0.1 μm, because reliability of the soldering of the semiconductorelement is increased.

[0095] For forming the electrically conductive pattern, commonly knownmethods can be used without any restriction. Examples of the methodspreferably used include a sputtering method, a vaporization method, achemical vapor phase deposition method (CVD), an ion plating method, amelt injection method, a screen printing method, and a sol-gel coatingmethod using spin coating or dip coating. For example, a metallic thinfilm to form the electrically conductive pattern is formed in thefollowing manner in accordance with the sputtering method. A targetcontaining a component of the metallic thin film is used, and thetemperature of the substrate is usually set in the range of roomtemperature to 300° C. After the vacuum vessel is evacuated to not morethan 2×10⁻³ Pa, an argon gas is introduced, then the vacuum vessel ismaintained at a pressure of 0.2 to 1.0 Pa, and the power of the RF(high-frequency) electric source is set in the range of 0.2 to 3 kW toform the metallic film having a desired thickness.

[0096] For the formation of a thin film composed of a nitride such asTa—N that is used for the resistor thin film pattern or a thin filmcomposed of an oxide such as Ta₂O₅ that is used for the capacitorpattern, a reactive sputtering method is preferably employed. Thereactive sputtering method means a method in which a target composed ofa metallic component of the objective compound is used, and a reactiongas containing another component of the objective compound such asnitrogen or oxygen is introduced into the vacuum tank together with anelectrical discharge gas to perform sputtering, whereby a thin film isobtained. The composition of the resulting thin film is determinedaccording to the ratio between the electrical discharge gas and thereaction gas introduced.

[0097] For example, a Ta—N film is formed in the following manner inaccordance with the reactive sputtering method. A target of Ta is used,and the temperature of the substrate is usually set in the range of roomtemperature to 300° C. After the vacuum vessel is evacuated to not morethan 2×10⁻³ Pa, an argon gas as the electrical discharge gas and anitrogen gas as the reaction gas are introduced, then the vacuum vesselis maintained at a pressure of 0.2 to 1.0 Pa, and the power of the RF(high-frequency) electric source is set in the range of 0.2 to 3 kW toform the metallic film having a desired thickness.

[0098] The shape of the electrically conductive pattern for use in theinvention can be arbitrarily selected according to the use purpose, andcan be made by patterning a metallic thin film for constituting theelectrically conductive pattern. For the patterning, any techniquecommonly known is adoptable according to the use purpose of thesubstrate. Specifically, a metal masking method, a wet etching method, alift-off method, a dry etching method or the like is adopted.

[0099] In the patterning by the metal masking method, a metal maskhaving a desired pattern previously formed is fixed on the substrate,and the aforesaid sputtering or vaporization is carried out to form anelectrically conductive pattern.

[0100] In the formation of the electrically conductive pattern by thedry etching method, a desired pattern using a photoresist or the like isformed on the metallic thin film having been formed on the substrate bythe aforesaid sputtering or vaporization, then the unnecessary portionof the metallic thin film is removed by ion milling or the like, andthen the resist is peeled off.

[0101] The method to form an electrically conductive pattern in whichthe circuit pattern contains a resistor thin film pattern is notspecifically restricted, and for example, the following methods areavailable. In one method, an electrically conductive pattern containinga connecting portion to be connected with a resistor thin film patternis formed on the substrate first. Then, on the electrically conductivepattern, a resistor thin film for forming a resistor thin film patternis laminated to form a resistor thin film pattern. According to thismethod, a circuit pattern in which the resistor thin film is laminatedon the electrically conductive pattern at the connecting portion can beobtained.

[0102] In another method, a resistor thin film pattern of a shapecontaining a connecting portion is previously formed on the substrate,and on the resistor thin film pattern, an electrically conductivepattern is formed. According to this method, a circuit pattern in whichthe electrically conductive pattern is laminated on the resistor thinfilm at the connecting portion can be obtained. Further, theelectrically conductive pattern containing a resistor thin film patterncan be formed in the following manner. That is, the resistor thin filmitself is used as the first layer that is brought into contact with thesubstrate. On the resistor thin film, a metallic thin film having anelectrical resistivity lower than that of the resistor thin film islaminated to form an electrically conductive pattern, and the metallicthin film formed on the resistor thin film is partially removed betweenthe specific patterns requiring the prescribed value of resistance ofthe electrically conductive pattern.

[0103] In order to suppress the change in the value of resistance due tothe lapse of time and temperature, the resistor thin film patternobtained as above is usually subjected to a treatment for stabilizingthe value of resistance (resistance stabilizing treatment) wherein anoxide film is formed on the surface of the resistor thin film. For theresistance stabilizing treatment, commonly known technique can be usedwithout any restriction. For example, formation of an oxide film by ananodizing method or formation of an oxide film by heating the substratehaving the resistor thin film pattern in the open air is carried out tostabilize the value of resistance. For adjusting the value of resistanceof the resulting resistor thin film pattern, commonly known techniquecan be used without any restriction. For example, laser trimming isavailable.

[0104] After the metallized substrate is plated with Ni, Au or the likein order to improve solder wettability, an electrode material such as aSi chip or pin can be soldered or brazed onto the substrate. As theplating means, electroless plating, electroplating, combination thereof,and the like can be used without any restriction.

[0105] The substrate of the invention may be machined into variousshapes by, for example, conducting grooving such as grinding or cutting.For example, the substrate may be subjected to slitting and then cuttingto give a convex chip as shown in the perspective view of FIG. 5. Thisconvex machined part is effectively used as a sub-mount, a chip carrieror the like.

[0106] As understood from the above description, the aluminum nitridesubstrate of the invention has a thermal conductivity of 190 W/mK ormore at 250C, and has an extremely high adhesion strength of 5.0 kg/mm²or more between the aluminum nitride sintering product and the internalelectrically conductive layer. Further, the substrate has no cracks inthe aluminum nitride sintering product and inside the via hole andhardly suffers warpage. Accordingly, the substrate of the invention isof extremely great value industrially.

[0107] If the aluminum nitride substrate of the invention having theabove properties is metallized to form a metallic thin film on thesubstrate surface, the resulting metallized substrate can be favorablyused for electronic parts or semiconductor parts, such as a sub-mount ora chip carrier of laser diode or light-emitting diode, a heat sink andan IC package.

EXAMPLE

[0108] The present invention is further described with reference to thefollowing examples, but it should be construed that the invention is inno way limited to those examples.

[0109] In the examples, various properties were measured by thefollowing methods.

[0110] (1) Carbon Residue in Aluminum Nitride Molded Product

[0111] The carbon residue was analyzed by a non-difusion type infraredray-absorption carbon analyzer (EMIA-110, manufactured by HoribaSeisakusho K.K.).

[0112] (2) Average Particle Diameter of Aluminum Nitride Powder

[0113] The average particle diameter D1 based on the specific surfacearea was calculated from the following formula.

D1(μm)=6/(S×3.26)

[0114] S: specific surface area of AlN powder (m²/g)

[0115] The average particle diameter D2 based on the sedimentationmethod was measured by a centrifugal particle sizedistribution-measuring device CAPA5000 manufactured by Horiba SeisakushoK.K.

[0116] (3) Evaluation of Appearance of Aluminum Nitride Substrate

[0117] The appearance was observed visually and by a stereomicroscope (×40), followed by evaluation.

[0118] A substrate in which no crack and no separation took place on theinterface between the aluminum nitride sintering product and theinternal electrically conductive layer, and no crack took place insidethe aluminum nitride sintering product, and no crack took place in theinternal electrically conductive layer was evaluated as good, and asubstrate in which at least one of these defects took place wasevaluated as bad.

[0119] (4) Warpage of Aluminum Nitride Substrate

[0120] The warpage was measured by a surface roughness and profilemeasuring device manufactured by Tokyo Seimitsu K.K.

[0121] (5) Thermal Conductivity of Aluminum Nitride Sintering Product

[0122] An aluminum nitride substrate of the same thickness, which wasprepared from the same materials by the same batchwise dewaxing andsintered but had no via hole and no internal electrically conductivelayer, was measured on the thermal conductivity by a laser flash method.

[0123] (6) Adhesion Strength Between Internal Electrically ConductiveLayer and Aluminum Nitride Sintering Product

[0124] The aluminum nitride substrate was polished like a mirror surfaceuntil the internal electrically conductive layer was exposed outside.Then, on the mirror surface of the substrate, sputtering operations ofTi of 0.06 μm, Pt of 0.2 μm and Au of 0.6 μm were carried out in thisorder to form a metallic thin film under the high vacuum of 10⁻³ Torr.The substrate was then cut to give a chip of about 5 mm×5 mm. On thetreated surface of the chip, a solder preform was placed, and on a hotplate at 230° C., a Ni-plated pin was perpendicularly soldered onto thesurface of the substrate. The pin has a flat tip, has a diameter of 0.5mm and is made of 42-alloy. The solder has a composition consisting of60% by weight of tin and 40% by weight of lead.

[0125] After soldering of the pin, the substrate was set in Strograph M2manufactured by Toyo Seiki Seisakusho K.K., and the pin was pulled inthe perpendicular direction to measure a breaking strength. The rate ofpulling was 10 mm/min. The position of the peel interface (peeling mode)was checked by observing the pin and the broken surface of the sinteringproduct after the test by the use of a stereomicroscope (× 40), ametallized microscope (× 400) and an X-ray microanalyzer.

[0126] (7) Adhesion Strength Between Electrically Conductive via Holeand Aluminum Nitride Sintering Product

[0127] The substrate was cut across the center of the via hole of thealuminum nitride sintering product, then the cut surface was polishedlike a mirror surface, and on the cut surface, a thin film of Ti/Pt/Auwas formed. Then, a Ni-plated pin was perpendicularly soldered so thatthe pin was brought into contact with the via hole surface. The pin hasa flat tip, has a diameter of 0.5 mm and is made of 42-alloy. The solderhas a composition consisting of 60% by weight of tin and 40% by weightof lead. The substrate was set in Strograph M2 manufactured by ToyoSeiki Seisakusho K.K., and the pin was pulled in the perpendiculardirection to measure a breaking strength. The rate of pulling was 10mm/min. The position of peel interface (peeling mode) was checked byobserving the pin and the broken surface of the sintering product afterthe test by the use of a stereomicroscope (× 40), a metallizedmicroscope (× 400) and an X-ray microanalyzer.

[0128] (8) Electrical Resistance of via Hole

[0129] The aluminum nitride substrate was ground to remove a portionabove or below the internal electrically conductive layer to allow thesubstrate to have a mirror surface. The substrate was then divided intosmall chips, and an electrical resistance of the via hole in each chipwas measured.

[0130] (9) Electrical Resistance Between Electrically Conductive Patternand via Hole

[0131] Needle-like probes were brought into contact with theelectrically conductive pattern on each surface of the metallizedsubstrate, said electrically conductive pattern being located rightabove the via hole penetrating the substrate. Then, an electricalresistance of the via hole of the metallized substrate was measured by afour-terminal method.

[0132] (10) Appearance of Metallized Substrate

[0133] The appearance of the metallized substrate was observed visuallyand by a stereomicroscope (× 40), followed by evaluation.

[0134] Whether disconnection of the electrically conductive pattern dueto cracks of the via hole or cracks of the aluminum nitride sinteringproduct had taken place or not was checked. A substrate having nodisconnection was evaluated as good, and a substrate having even onedisconnection was evaluated as bad.

Example 1

[0135] 100 Parts by weight of an aluminum nitride powder (averageparticle diameter based on the sedimentation method: 1.50 μm, specificsurface area: 2.50 m²/g, average particle diameter calculated from thespecific surface area: 0.74 μm, oxygen content: 0.80%) having acomposition shown in Table 1, 5 parts by weight of yttoria, 2 parts byweight of n-butyl methacrylate as a dispersant, 11 parts by weight ofpolybutyl acrylate as an organic binder, 7 parts by weight of dioctylphthalate as a plasticizer and 50 parts by weight of a mixed solvent oftoluene and isopropyl alcohol were weighed, and they were introducedinto a ball milling pot, followed by sufficiently mixing by the use of anylon ball. TABLE 1 Analytic value of AIN powder AlN content 97.9%Element Content Ca 105 ppm Si  63 ppm Fe  12 ppm Ti  16 ppm V  0.8 ppm O 0.80% C 0.03%

[0136] The resulting slurry was introduced into a defoaming apparatus soas to have a viscosity of 20000 cps, and using the slurry, a sheet wasformed on a polypropylene film by a doctor blade type sheet-formingmachine to prepare an aluminum nitride green sheet having a thickness ofabout 0.40 mm.

[0137] The aluminum nitride green sheet was cut into a size of 65×65 mm.Then, three of the aluminum nitride green sheets were laminated togetherto prepare an aluminum nitride molded product (I). The laminatingpressure was 50 kgf/cm², the laminating temperature was 80° C., and thelaminating time was 15 minutes.

[0138] Then, the aluminum nitride molded product (I) (65×65 mm) waspunched by a punching metal mold having a diameter of 0.28 mmmaintaining a pitch of 0.5 mm in order to form through holes arranged ina number of 120×120. Separately, to 100 parts by weight of a tungstenpowder (average particle diameter based on the Fischer's method: 1.8 μm)were added 5 parts by weight of the aluminum nitride powder, 1.5 partsby weight of ethyl cellulose as an organic binder, 5.0 parts by weightof 2-(2-butoxyethoxy)ethyl acetate as a solvent and some amounts of aplasticizer and a dispersant, and they were sufficiently kneaded bymeans of an automatic mortar and then a roll mill having three rolls toobtain a paste (paste (A) for forming electrically conductive via hole)having a viscosity of 2300 poise at 25° C./5 rpm. The concentration ofthe refractory metal in the paste (A) was 90.8% by weight. The paste (A)was filled in the through holes of the aluminum nitride molded product(I) by the pressurized penetration method. The filling pressure was 50psi, and the filling time was 120 seconds.

[0139] Then, to 100 parts by weight of a tungsten powder (averageparticle diameter based on the Fischer's method: 2.5 μm) were added 5parts by weight of the aluminum nitride powder, 2 parts by weight ofethyl cellulose, 20 parts by weight of terpineol and some amounts of aplasticizer and a dispersant, and they were sufficiently kneaded bymeans of a grinding mill and then a roll mill having three rolls toobtain a paste (paste (B) for forming internal electrically conductivelayer) having a viscosity of 950 poise at 25° C./5 rpm. Theconcentration of the refractory metal in the paste (B) was 79.5% byweight. The paste (B) was printed on one surface of the aluminum nitridemolded product (I) by screen printing to form a whole surface solidpattern, whereby an electrically conductive paste layer was formed onone surface of the aluminum nitride molded product (I). After drying,the thickness of the electrically conductive layer was measured by adepth indicator, and as a result, the thickness was 20 μm.

[0140] Another aluminum nitride green sheet (II) cut into 65×65 mm waspunched by a punching metal mold having a diameter of 0.28 mmmaintaining a pitch of 1.5 mm in order to form through holes arranged ina number of 40×40. The through holes were filled with the paste (A) bythe pressurized penetration method to prepare an aluminum nitride moldedproduct (II). The filling pressure was 650 psi, and the filling time was180 seconds.

[0141] Then, the aluminum nitride molded product (II) was laminated onthe electrically conductive paste layer side surface of the aluminumnitride molded product (I). The laminating pressure was 80 kgf/cm², thelaminating temperature was 80° C., and the laminating time was 15minutes.

[0142] The resulting aluminum nitride molded product laminate (i) havingvia hole-forming through holes filled with the paste (A) and inside anelectrically conductive paste layer was dewaxed under heating at 900° C.for 2 hours with passing a dry nitrogen gas at a rate of 30 1/min. Thetemperature was elevated at a rate of 2.5° C./min. A test sample havingbeen dewaxed under heating at the same time was examined on the carbonresidue, and as a result, the carbon residue was 2430 ppm. Afterdewaxing, the dewaxed laminate was placed in an aluminum nitridecontainer, heated at 1580° C. for 6 hours in a nitrogen atmosphere(first step firing) and then heated at 1870° C. for 10 hours in anitrogen atmosphere (second step firing). Thus, an aluminum nitridesubstrate having a thickness of 1.2 mm and a structure shown in thesectional view of FIG. 1 was prepared.

[0143] Various properties of the aluminum nitride substrate obtainedwere measured. The results are set forth in Table 3.

Examples 2 and 3, Comparative Examples 1 and 2

[0144] An aluminum nitride substrate was prepared in the same manner asin Example 1, except that the composition of the paste (A) (electricallyconductive paste for forming via hole) was changed as shown in Table 2.Various properties of the aluminum nitride substrate were measured. Theresults are set forth in Table 3.

Examples 4 and 5, Comparative Example 3

[0145] An aluminum nitride substrate was prepared in the same manner asin Example 1, except that the composition of the paste (B) (electricallyconductive paste for forming internal electrically conductive layer) waschanged as shown in Table 2. Various properties of the aluminum nitridesubstrate were measured. The results are set forth in Table 3.

Example 6

[0146] The aluminum nitride molded product laminate (i) prepared inExample 1 was dewaxed under heating at 850° C. for 2 hours with passinga dry hydrogen gas at a rate of 12 l/min. The temperature was elevatedat a rate of 2.5° C./min. A test sample having been dewaxed underheating at the same time was examined on the carbon residue, and as aresult, the carbon residue was 900 ppm. After dewaxing, the dewaxedlaminate was sintered in the same manner as in Example 1 to prepare analuminum nitride substrate. Various properties of the aluminum nitridesubstrate were measured. The results are set forth in Table 3.

Example 7

[0147] The aluminum nitride molded product laminate (i) prepared inExample 1 was dewaxed under heating at 900° C. for 2 hours with passinga dry nitrogen gas at a rate of 20 l/min. The temperature was elevatedat a rate of 2.5° C./min. A test sample having been dewaxed underheating at the same time was examined on the carbon residue, and as aresult, the carbon residue was 2800 ppm. After dewaxing, the dewaxedlaminate was sintered in the same manner as in Example 1 to prepare analuminum nitride substrate. Various properties of the aluminum nitridesubstrate were measured. The results are set forth in Table 3.

Comparative Example 4

[0148] The aluminum nitride molded product laminate (i) prepared inExample 1 was dewaxed under heating at 850° C. for 2 hours with passinga moistened hydrogen gas at a rate of 10 l/min. The temperature waselevated at a rate of 2.5° C./min. A test sample having been dewaxedunder heating at the same time was examined on the carbon residue, andas a result, the carbon residue was 600 ppm. After dewaxing, the dewaxedlaminate was sintered in the same manner as in Example 1 to prepare analuminum nitride substrate. Various properties of the aluminum nitridesubstrate were measured. The results are set forth in Table 3.

Comparative Example 5

[0149] The aluminum nitride molded product laminate (i) prepared inExample 1 was dewaxed under heating at 900° C. for 2 hours with passinga dry nitrogen gas at a rate of 15 l/min. The temperature was elevatedat a rate of 2.5° C./min. A test sample having been dewaxed underheating at the same time was examined on the carbon residue, and as aresult, the carbon residue was 3500 ppm. After dewaxing, the dewaxedlaminate was sintered in the same manner as in Example 1 to prepare analuminum nitride substrate. Various properties of the aluminum nitridesubstrate were measured. The results are set forth in Table 3. TABLE 2Amount of AlN added Concentration of Viscosity of paste Firing FiringCorresponding Carbon (part(s) by weight) refractory metal (poise)temperature temperature sectional residue Paste Paste (% by weight)Paste Paste (° C.) (° C.) view (ppm) (A) (B) Paste (A) Paste (B) (A) (B)First step Second step Ex. 1 2430 5 5 90.8 79.5 2300 950 1580 1870 Ex. 22430 3 5 92.6 79.5 1980 950 1580 1870 Ex. 3 2430 9 5 87.5 79.5 2140 9501580 1870 Comp. 2430 0 5 96.5 79.5 3500 950 1580 1870 Ex. 1 Comp. 243013 5 82.5 79.5 4710 950 1580 1870 Ex. 2 Ex. 4 2200 5 12 90.8 71.3 23001000 1580 1870 Ex. 5 2010 5 18 90.8 65.7 2300 1030 1580 1870 Comp. 25305 0 90.8 84.7 2300 770 1580 1870 Ex. 3 Ex. 6 900 5 5 90.8 79.5 2300 9501580 1870 Ex. 7 2800 5 5 90.8 79.5 2300 950 1580 1870 Comp. 600 5 5 90.879.5 2300 950 1580 1870 Ex. 4 Comp. 3500 5 5 90.8 79.5 2300 950 15801870 Ex. 5

[0150] TABLE 3 Internal electrically Electrically conductive Warpage ofThermal conductive layer/AlN via hole/AlN Appearance sinteringconductivity Adhesion Adhesion Electrical of sintering product ofsintering strength Peeling strength Peeling resistance of product (μm)product (W/mK) (kg/mm²) mode (kg/mm²) mode via hole (mΩ) Ex. 1 good 50215 16.4 in solder 13.4 in solder 8.0/8.1 Ex. 2 good 52 211 16.3 insolder 12.2 in solder 4.2/4.3 Ex. 3 good 49 209 16.1 in solder 16.3 insolder 15.8/15.9 Comp. bad 50 205 3.1 W/AlN 1.7 W/AlN 4.0/4.1 Ex. 1Comp. bad 48 207 2.2 W/AlN 2.4 W/AlN 54.4/55.0 Ex. 2 Ex. 4 good 65 21217.0 in solder 13.1 in solder 7.2/7.3 Ex. 5 good 78 208 17.1 in solder12.8 in solder 6.5/6.6 Comp. bad 47 217 1.7 W/AlN 5.2 in solder 8.3/8.4Ex. 3 Ex. 6 good 18 197 13.5 in solder 10.8 in solder 5.2/5.3 Ex. 7 good72 218 15.6 in solder 12.8 in solder 15.7/16.0 Comp. good 15 162 8.2W/AlN 6.2 W/AlN 4.5/4.8 Ex. 4 Comp. bad 262 195 3.3 W/AlN 2.4 W/AlN46.0/47.5 Ex. 5

Examples 8-11, Comparative Examples 6-9

[0151] An aluminum nitride substrate was prepared in the same manner asin Example 1, except that the firing conditions of the aluminum nitridemolded product laminate (i) were changed as shown in Table 4. Variousproperties of the aluminum nitride substrate were measured. The resultsare set forth in Table 5.

Example 12

[0152] The aluminum nitride green sheet prepared in Example 1 was cutinto a size of 65×65 mm. The aluminum nitride green sheet was laminatedon an electrically conductive paste layer side surface of an aluminumnitride molded product (I) prepared in the same manner as in Example 1.The laminating pressure was 80 kgf/cm², the laminating temperature was75° C., and the laminating time was 15 minutes.

[0153] The resulting aluminum nitride molded product laminate (ii) wasdewaxed under heating under the same conditions as in Example 1. A testsample having been dewaxed at the same time was examined on the carbonresidue, and as a result, the carbon residue was 2030 ppm. Afterdewaxing, the dewaxed laminate was placed in an aluminum nitridecontainer and sintered under the same conditions as in Example 1.

[0154] The resulting aluminum nitride substrate having a structure shownin the sectional view of FIG. 2 was measured on various properties. Theresults are set forth in Table 5.

Example 13

[0155] On one surface of the aluminum nitride molded product laminate(i) having been prepared in Example 1 and having via hole-forming holesfilled with the paste (A) and inside an electrically conductive pastelayer, the paste (B) was printed by screen printing to form a wholesurface solid pattern. The printed film thickness was 18 μm.

[0156] Separately, another aluminum nitride green sheet cut into 65×65mm was punched by a punching metal mold having a diameter of 0.28 mmmaintaining a pitch of 1.0 mm in order to form through holes arranged ina number of 60×60. The through holes were filled with the paste (A) bythe pressurized penetration method to prepare an aluminum nitride moldedproduct (III). The filling pressure was 55 psi, and the filling time was150 seconds.

[0157] Then, the aluminum nitride molded product (III) was laminated onthe electrically conductive paste layer side surface of the aluminumnitride molded product laminate (i). The laminating pressure was 85kgf/cm², the laminating temperature was 70° C., and the laminating timewas 15 minutes.

[0158] Thus, an aluminum nitride molded product laminate (iii) havinginside two electrically conductive paste layers was prepared. Thealuminum nitride molded product laminate (iii) was dewaxed under heatingunder the same conditions as in Example 1. A test sample having beendewaxed under heating at the same time was examined on the carbonresidue, and as a result, the carbon residue was 2890 ppm. Afterdewaxing, the dewaxed laminate was placed in an aluminum nitridecontainer and sintered under the same conditions as in Example 1. Thus,an aluminum nitride substrate having a thickness of 1.5 mm and astructure shown in the sectional view of FIG. 3 was prepared.

[0159] Various properties of the aluminum nitride substrate obtainedwere measured. The results are set forth in Table 5.

Example 14

[0160] An aluminum nitride substrate having a structure shown in thesectional view of FIG. 4 was prepared in the same manner as in Example1, except that the whole surface solid pattern formed on one surface ofthe aluminum nitride molded product (I) by screen printing of the paste(B) was changed to a circuit pattern, and the number and the positionsof the through holes to be formed in the aluminum nitride molded product(II) were changed to those corresponding to the above circuit pattern.

[0161] Various properties of the aluminum nitride substrate obtainedwere measured. The results are set forth in Table 5.

[0162] Then, the aluminum nitride substrate was polished like a mirrorsurface so that the surface coarseness Ra became 0.02 μm, and then thesubstrate was slit from the upper surface of the substrate to the depthof 0.15 mm under the conditions of a width of 1.2 mm and a pitch of 4.2mm to give a shape having a convex section. The resulting substrate wasthen cut into a chip having a width of 2.0 mm and a length of 4.0 mm.

[0163] Thus, there was prepared such a substrate of convex shape asshown in the perspective view of FIG. 5 wherein the internalelectrically conductive layer was formed as a circuit pattern having anaverage line width of 0.1 mm and a minimum line width of 0.080 mm, theproportion of the internal electrically conductive layer to thehorizontal section of the substrate was 68%, and six via holespenetrating from the upper surface of the convex portion from the lowersurface of the base portion were formed. TABLE 4 Amount of AlN addedFiring temperature Firing temperature Corresponding Carbon residue(part(s) by weight) (° C.) (° C.) sectional view (ppm) Paste (A) Paste(B) First step Second step Ex. 8 2430 5 5 1250 1870 Ex. 9 2430 5 5 16501870 Ex. 10 2430 5 5 1580 1820 Ex. 11 2430 5 5 1580 1930 Comp. 2430 5 51100 1870 Ex. 6 Comp. 2430 5 5 1800 1870 Ex. 7 Comp. 2430 5 5 1580 1750Ex. 8 Comp. 2430 5 5 1580 2000 Ex. 9 Ex. 12 2030 5 5 1580 1870 Ex. 132890 5 5 1580 1870 Ex. 14 1870 5 5 1580 1870

[0164] TABLE 5 Electrically Thermal Internal electrically conductive viaWarpage of conductivity conductive layer/AlN hole/AlN Appearancesintering of sintering Adhesion Adhesion Electrical of sintering productproduct strength mode strength mode resistance of product (μm) (W/mK)(kg/mm²) mode (kg/mm²) mode via hole (mΩ) Ex. 8 good 48 209 16.2 insolder 13.2 in solder 18.5/18.7 Ex. 9 good 50 211 16.4 in solder 13.4 insolder 8.1/8.1 Ex. 10 good 42 207 16.1 in solder 13.5 in solder 9.0/9.1Ex. 11 good 60 220 16.2 in solder 12.0 in solder 6.3/6.4 Comp. good 49170 10.4 in solder 8.5 in solder 65.0/68.0 Ex. 6 Comp. good 44 171 13.2in solder 10.8 in solder 16.0/16.5 Ex. 7 Comp. bad 45 153 7.9 in solder6.5 W/AlN 12.0/13.5 Ex. 8 Comp. bad 287 205 2.4 W/AlN 2.0 W/AlN21.0/21.3 Ex. 9 Ex. 12 good 42 210 16.9 in solder 13.1 in solder  —/6.3Ex. 13 good 87 219 15.9/15.1 in solder 13.8 in solder 15.8/15.6/16.0 Ex.14 good 27 215 15.2 in solder 13.3 in solder 6.0/6.1

Examples 15-21, Comparative Examples 10-14

[0165] An electrically conductive pattern was formed on each surface ofthe substrate obtained in each of Examples 1 to 7 and ComparativeExamples 1 to 5 in the following manner to obtain a metallizedsubstrate.

[0166] In the first place, the surface of the substrate was ground bythe use of diamond abrasive grains until the substrate thickness became1.2 mm and the sintering product had a mirror surface. For the grinding,the side end of the internal electrically conductive layer was exposedoutside on the side surface of the substrate, and the grinding wascarried out so that the distance between the side end of the internalelectrically conductive layer and the substrate surface became 0.2 mm.The centerline average coarseness (Ra) on the surface of the resultingsubstrate was measured by a Surfcom 550A manufactured by Tokyo SeimitsuK.K., and as a result, Ra was 0.02 μm.

[0167] The thus treated substrate was subjected to ultrasonic cleaningin methylene chloride, then dried in vapor of methylene chloride andsubjected to sputtering to form metallic thin films (first layer/secondlayer/third layer=Ti: 0.1 μm/Pt: 0.2 μm/Au: 0.5 μm) all over the frontand back surfaces of the substrate. After formation of the metallic thinfilms, the metallic thin films on the front and the back surfaces weresubjected to dry etching to form electrically conductive patterns.

[0168] In the next place, a Ta—N thin film having a thickness of 0.1 μmwas formed all over the front surface of the substrate including theelectrically conductive pattern by a reactive sputtering method. TheTa—N thin film was analyzed by a fully automatic X-ray diffractionapparatus manufactured by Nippon Philips K.K., and as a result, adiffraction peak of a Ta₂N phase was observed. After the formation ofthe Ta—N thin film, an unnecessary portion of the Ta—N thin film wasremoved by a wet etching method to form a resistor thin film pattern.After the formation of the resistor thin film layer, the substrate washeated in the open air at 360° C. for 4 hours to perform a resistancestabilizing treatment.

[0169] The resulting metallized substrate was cut into a chip by adicing machine to obtain a metallized substrate.

[0170] Various properties of the metallized substrate obtained weremeasured. The results are set forth in Table 6. TABLE 6 Electricalresistance between Appearance of electrically conductive patternmetallized and via hole Substrate substrate (mΩ) Ex. 1 good 48.0/48.6Ex. 2 good 25.6/26.1 Ex. 3 good 78.2/79.2 Comp. bad 24.0/24.6 Ex. 1Comp. bad 321/330 Ex. 2 Ex. 4 good 43.8/44.6 Ex. 5 good 38.2/37.2 Comp.bad 51.2/53.4 Ex. 3 Ex. 6 good 30.8/31.3 Ex. 7 good 82.4/83.1 Comp. good28.2/28.8 Ex. 4 Comp. bad 277/291 Ex. 5

What is claimed is:
 1. A substrate formed from an aluminum nitridesintering product having an internal electrically conductive layer, atleast one electrically conductive via hole formed between the internalelectrically conductive layer and at least one surface of the substrate,wherein: the thermal conductivity of the aluminum nitride sinteringproduct at 25° C. is 190 W/mK or more, and the adhesion strength betweenthe aluminum nitride sintering product and the internal electricallyconductive layer is 5.0 kg/mm² or more.
 2. The substrate as claimed inclaim 1, wherein the adhesion strength between the aluminum nitridesintering product and the electrically conductive via hole is 5.0 kg/mm²or more.
 3. The substrate as claimed in claim 1 or 2, wherein theelectrically conductive via hole comprises a sintering product of anelectrically conductive paste having a refractory metal concentration of85 to 95% by weight, and the internal electrically conductive layercomprises a sintering product of an electrically conductive paste havinga refractory metal concentration of 65 to 83% by weight.
 4. A metallizedsubstrate having an electrically conductive pattern formed on at leastone surface of both surfaces of the substrate of any one of claims 1 to3, wherein at least a part of the electrically conductive pattern iselectrically connected to the electrically conductive via hole.
 5. Aprocess for producing a substrate, comprising: forming at least one viahole-forming through hole in a first aluminum nitride molded productcomprising an aluminum nitride powder, a sintering aid and an organicbinder, filling the through hole with an electrically conductive paste(A) comprising 100 parts by weight of a refractory metal powder and 2 to10 parts by weight of an aluminum nitride powder, coating the surface ofthe first aluminum nitride molded product with an electricallyconductive paste (B) comprising 100 parts by weight of a refractorymetal powder and 2 to 20 parts by weight of an aluminum nitride powderto form an electrically conductive paste layer, laminating a secondaluminum nitride molded product comprising an aluminum nitride powder, asintering aid and an organic binder on the first aluminum nitride moldedproduct through the layer of the electrically conductive paste (B), anddewaxing the resulting aluminum nitride molded product laminate so thatthe carbon residue becomes 800 to 3000 ppm, then firing the laminate ata temperature of 1200 to 1700° C. and further firing the laminate at atemperature of 1800 to 1950° C.
 6. The process for producing a substrateas claimed in claim 5, wherein the concentration of the refractory metalin the electrically conductive paste (A) with which the via hole-formingthrough hole of the first aluminum nitride molded product is to befilled is in the range of 85 to 95% by weight, and the concentration ofthe refractory metal in the electrically conductive paste (B) with whichthe surface of the first aluminum nitride molded product is to be coatedis in the range of 65 to 83% by weight.
 7. The process for producing asubstrate as claimed in claim 6, wherein the viscosity of theelectrically conductive paste (A) with which the via hole-formingthrough hole of the first aluminum nitride molded product is to befilled is in the range of 100 to 30000 poise at 25° C./5 rpm, and theviscosity of the electrically conductive paste (B) with which thesurface of the first aluminum nitride molded product is to be coated isin the range of 800 to 1200 poise at 25° C./5 rpm.
 8. A substrate formedfrom an aluminum nitride sintering product obtainable by the process ofany one of claims 5 to 7, having an internal electrically conductivelayer, having at least one electrically conductive via hole formedbetween the internal electrically conductive layer and at least onesurface of the substrate, wherein: the thermal conductivity of thealuminum nitride sintering product at 25° C. is 190 W/mK or more, andthe adhesion strength between the aluminum nitride sintering product andthe internal electrically conductive layer is 5.0 kg/mm² or more.